The present invention relates to a novel process for loading metals onto adsorbent beds in an overall adsorption process unit comprising at least two beds, an adsorption bed and either an upstream bed (e.g. a pretreatment bed) or a downstream bed (e.g. a post treatment bed). The method is particularly useful for in-situ ion exchange loading of metals for which the adsorbent has a strong affinity.
Metal loaded adsorbents are applied in a wide variety of separation and contaminant removal processes. For example, U.S. Pat. No. 4,874,525 B1 teaches the use of a zeolite containing either ionic or elemental silver to remove mercury contaminants from a number of process streams including natural gas. U.S. Pat. No. 5,013,334 B1 employs a zinc metal-exchanged faujasite type of zeolitic aluminosilicate to effect the separation of ethane from methane by selective adsorption. U.S. Pat. No. 4,529,828 B1 provides a silver cation exchanged zeolite X for the isolation of ortho-xylene from a stream of mixed xylene isomers. For purifying commercial acetic acid streams produced by methanol carbonylation in the presence of an iodide promoter, both zeolites and resins that have been exchanged with an iodide reactive metal (e.g. silver) can be employed, as disclosed in U.S. Pat. No. 5,962,735 B1 and U.S. Pat. No. 4,615,806 B1, respectively.
The commercial preparation of metal loaded adsorbents may involve agitating or stirring a slurry of adsorbent particles in a metal containing solution, such as an aqueous solution of a salt of the metal. Otherwise, the adsorbent may be placed in a fixed bed and contacted continuously with a metal-containing solution. In this mode of operation, recycling of the solution over the adsorbent to increase the solution residence time is often desirable. In any case, the main contacting conditions of time, temperature, solution concentration, and agitation rate are based on a number of factors, including the equilibrium ratio of solid-phase to dissolved metal, the rate of metal uptake, the desired concentration of loaded metal, and the structural integrity of the adsorbent. Metal loading may be enhanced, especially in cases where the metal used is limited in solubility in the solution, by periodically or continuously withdrawing a portion of solution that has been depleted of its metal content during the process of metal loading the adsorbent. The withdrawn solution is then replaced with fresh solution having a higher metal content than that removed.
In general, commercial methods for loading metals by ion exchange, impregnation, pore filling, and others suffer in that waste streams are generated that normally require further treatment. In some cases, residual metal solutions, for example those containing mercury or lead, can pose environmental hazards without special handling and disposal techniques. The use of certain metallic solutions may be precluded altogether at certain adsorbent preparation facilities lacking necessary permits and/or recovery equipment. Furthermore, even when the metal loading of adsorbents is authorized, expensive metals such as silver, platinum, and palladium must be recovered from waste streams, at some cost, and re-used.
While the metal loading of adsorbents according to methods described above results in a wet material, the efficient transport of such a metal loaded adsorbent mandates that it be shipped in a dry state. This also economically impacts the synthesis of metal loaded adsorbents in terms of higher costs for energy and disposal associated with driving metal containing solution from the adsorbent. Finally, with resins in particular, material shrinkage may significantly reduce the volume of dry material, complicating commercial bed designs where the adsorbent is used in its swollen state.
To overcome these drawbacks associated with prior art methods of preparing metal loaded adsorbents in separate manufacturing facilities, followed by transport to the location of their use, applicant now provides an effective method for in-situ loading of metal onto an adsorbent. The method is applicable for the significant number of commercial adsorption processes that rely on adsorbent beds for either pre- or post-treatment in addition to the primary adsorption function. These supplemental beds are often not loaded initially with metal, but can serve to guard against poisons and foulants that detrimentally affect the overall adsorption process. Alternatively, adsorbent beds downstream of the initially metal loaded bed serve to trap or contain valuable metal that can leach from the metal loaded adsorbent bed over the course of normal operation. A characteristic of supplemental adsorbent beds to which the present invention applies is that they are not significantly affected, in terms of their intended purpose, when partially loaded with some the same type of metal that is loaded onto the adsorption bed.
The present invention is a method of loading metal onto at least one adsorption bed of adsorbent and at least a second bed of adsorbent used for either pre- or post-treatment of the process stream to be treated by adsorption. The invention overcomes many of the drawbacks associated with preparing metal loaded adsorbents at a location remote from the process where such adsorbents are ultimately used. A specific advantage of the invention is the convenient, in-situ treatment of residual streams that would require separate disposal in commercial adsorbent manufacturing facilities.
In one embodiment, the present invention is a method for in-situ loading of a metal on an adsorbent system comprising an adsorption bed and a post-treatment bed downstream of the adsorption bed in normal operation. The adsorption bed and post-treatment bed have both affinity and capacity for the metal. The method comprises flowing a solution of the metal through the adsorption bed to contact it with the solution and yield a metal loaded adsorption bed, a first metal-depleted solution within the adsorption bed, and excess solution. The method further comprises contacting at least part of the post-treatment bed with the excess solution to load at least a portion of the metal contained therein onto the post-treatment bed and yield second metal-depleted solution within the post-treatment bed. The method further comprises transferring the first metal-depleted solution from the metal loaded adsorption bed to either the post-treatment bed or a pretreatment bed to load at least a portion of the metal contained in the first metal-depleted solution thereon.
In a more specific embodiment, the present invention is a method as described above where the solution is an aqueous solution of a compound of the metal and an anion selected from the group consisting of nitrate, nitrite, sulfate, sulfite, phosphate, carbonate, acetate, hydroxide, and mixtures thereof; the adsorption bed comprises a zeolite molecular sieve selected from the group consisting of LZ-210, Y-85, mordenite, zeolite A, zeolite X, zeolite Y, and mixtures thereof; and the metal is selected from the group consisting of silver, mercury, copper, lead, thallium, palladium, barium, and mixtures thereof; where the metal is loaded onto the adsorption bed by ion exchange.
In another embodiment, the present invention is a method for in-situ loading of a metal on an adsorbent system comprising a pretreatment bed and an adsorption bed downstream of the pretreatment bed in normal operation. The pretreatment bed and adsorption bed have both affinity and capacity for the metal. The method comprises flowing a solution of the metal through the adsorption bed to contact it with the solution and yield a metal loaded adsorption bed, a first metal-depleted solution within the adsorption bed, and excess solution. The method further comprises contacting at least part of the pretreatment bed with the excess solution to load at least a portion of the metal contained therein onto the pretreatment bed and yield second metal-depleted solution within the pretreatment bed. The method further comprises transferring the first metal-depleted solution from the metal loaded adsorption bed to either the pretreatment bed or a post-treatment bed to load at least a portion of the metal contained in the first metal-depleted solution thereon.
In yet another embodiment, the present invention is a method according to the last-mentioned embodiment and further comprising removing the second metal-depleted solution from the pretreatment bed to the post-treatment bed to load at least a portion of the metal contained in the second metal-depleted solution thereon.
These and other embodiments and objects will become clearer after the detailed description of the invention.